Chapter 17: Alcohols and Phenols Based on McMurry’s Organic Chemistry, 6th edition ©2003 Ronald Kluger Department of Chemistry University of Toronto

Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003 Alcohols and Phenols Alcohols contain an OH group connected to a a saturated C (sp3) They are important solvents and synthesis intermediates Phenols contain an OH group connected to a carbon in a benzene ring Methanol, CH3OH, called methyl alcohol, is a common solvent, a fuel additive, produced in large quantities Ethanol, CH3CH2OH, called ethyl alcohol, is a solvent, fuel, beverage Phenol, C6H5OH (“phenyl alcohol”) has diverse uses - it gives its name to the general class of compounds Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003 17.1 Naming Alcohols General classifications of alcohols based on substitution on C to which OH is attached Methyl (C has 3 H’s), Primary (1°) (C has two H’s, one R), secondary (2°) (C has one H, two R’s), tertiary (3°) (C has no H, 3 R’s), Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

IUPAC Rules for Naming Alcohols Select the longest carbon chain containing the hydroxyl group, and derive the parent name by replacing the -e ending of the corresponding alkane with -ol Number the chain from the end nearer the hydroxyl group Number substituents according to position on chain, listing the substituents in alphabetical order Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Many Alcohols Have Common Names These are accepted by IUPAC Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003 Naming Phenols Use “phene” (the French name for benzene) as the parent hydrocarbon name, not benzene Name substituents on aromatic ring by their position from OH Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

17.2 Properties of Alcohols and Phenols: Hydrogen Bonding The structure around O of the alcohol or phenol is similar to that in water, sp3 hybridized Alcohols and phenolshave much higher boiling points than similar alkanes and alkyl halides Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Alcohols Form Hydrogen Bonds A positively polarized OH hydrogen atom from one molecule is attracted to a lone pair of electrons on a negatively polarized oxygen atom of another molecule This produces a force that holds the two molecules together These intermolecular attractions are present in solution but not in the gas phase, thus elevating the boiling point of the solution Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

17.3 Properties of Alcohols and Phenols: Acidity and Basicity Weakly basic and weakly acidic Alcohols are weak Brønsted bases Protonated by strong acids to yield oxonium ions, ROH2+ Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Alchols and Phenols are Weak Brønsted Acids Can transfer a proton to water to a very small extent Produces H3O+ and an alkoxide ion, RO, or a phenoxide ion, ArO Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Brønsted Acidity Measurements The acidity constant, Ka, measure the extent to which a Brønsted acid transfers a proton to water [A] [H3O+] Ka = ————— and pKa = log Ka [HA] Relative acidities are more conveniently presented on a logarithmic scale, pKa, which is directly proportional to the free energy of the equilibrium Differences in pKa correspond to differences in free energy Table 17.1 presents a range of acids and their pKa values Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

pKa Values for Typical OH Compounds Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Relative Acidities of Alcohols Simple alcohols are about as acidic as water Alkyl groups make an alcohol a weaker acid The more easily the alkoxide ion is solvated by water the more its formation is energetically favored Steric effects are important Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003 Inductive Effects Electron-withdrawing groups make an alcohol a stronger acid by stabilizing the conjugate base (alkoxide) Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Generating Alkoxides from Alcohols Alcohols are weak acids – requires a strong base to form an alkoxide such as NaH, sodium amide NaNH2, and Grignard reagents (RMgX) Alkoxides are bases used as reagents in organic chemistry Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003 Phenol Acidity Phenols (pKa ~10) are much more acidic than alcohols (pKa ~ 16) due to resonance stabilization of the phenoxide ion Phenols react with NaOH solutions (but alcohols do not), forming soluble salts that are soluble in dilute aqueous A phenolic component can be separated from an organic solution by extraction into basic aqueous solution and is isolated after acid is added to the solution Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003 Substituted Phenols Can be more or less acidic than phenol itself An electron-withdrawing substituent makes a phenol more acidic by delocalizing the negative charge Phenols with an electron-donating substituent are less acidic because these substituents concentrate the charge Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003 Nitro-Phenols Phenols with nitro groups at the ortho and para positions are much stronger acids The pKa of 2,4,6-trinitrophenol is 0.6, a very strong acid Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

17.4 Preparation of Alchols: an Overview Alcohols are derived from many types of compounds The alcohol hydroxyl can be converted to many other functional groups This makes alcohols useful in synthesis Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Review: Preparation of Alcohols by Regiospecific Hydration of Alkenes Hydroboration/oxidation: syn, non-Markovnikov hydration Oxymercuration/reduction: Markovnikov hydration Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003 Preparation of 1,2-Diols Review: Cis 1,2-diols from hydroxylation of an alkene with OsO4 followed by reduction with NaHSO3 In Chapter 18: Trans-1,2-diols from acid-catalyzed hydrolysis of epoxides Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

17.5 Alcohols from Reduction of Carbonyl Compounds Reduction of a carbonyl compound in general gives an alcohol Note that organic reduction reactions add the equivalent of H2 to a molecule Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Reduction of Aldehydes and Ketones Aldehydes gives primary alcohols Ketones gives secondary alcohols Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Reduction Reagent: Sodium Borohydride NaBH4 is not sensitive to moisture and it does not reduce other common functional groups Lithium aluminum hydride (LiAlH4) is more powerful, less specific, and very reactive with water Both add the equivalent of “H-” Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Mechanism of Reduction The reagent adds the equivalent of hydride to the carbon of C=O and polarizes the group as well Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Reduction of Carboxylic Acids and Esters Carboxylic acids and esters are reduced to give primary alcohols LiAlH4 is used because NaBH4 is not effective Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003 17.6 Alcohols from Reaction of Carbonyl Compounds with Grignard Reagents Alkyl, aryl, and vinylic halides react with magnesium in ether or tetrahydrofuran to generate Grignard reagents, RMgX Grignard reagents react with carbonyl compounds to yield alcohols Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Examples of Reactions of Grignard Reagents with Carbonyl Compounds Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Reactions of Esters and Grignard Reagents Yields tertiary alcohols in which two of the substituents carbon come from the Grignard reagent Grignard reagents do not add to carboxylic acids – they undergo an acid-base reaction, generating the hydrocarbon of the Grignard reagent Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Grignard Reagents and Other Functional Groups in the Same Molecule Can't be prepared if there are reactive functional groups in the same molecule, including proton donors Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Mechanism of the Addition of a Grignard Reagent Grignard reagents act as nucleophilic carbon anions (carbanions, : R) in adding to a carbonyl group The intermediate alkoxide is then protonated to produce the alcohol Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

17.7 Some Reactions of Alcohols Two general classes of reaction At the carbon of the C–O bond At the proton of the O–H bond Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Dehydration of Alcohols to Yield Alkenes The general reaction: forming an alkene from an alcohol through loss of O-H and H (hence dehydration) of the neighboring C–H to give  bond Specific reagents are needed Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Acid- Catalyzed Dehydration Tertiary alcohols are readily dehydrated with acid Secondary alcohols require severe conditions (75% H2SO4, 100°C) - sensitive molecules don't survive Primary alcohols require very harsh conditions – impractical Reactivity is the result of the nature of the carbocation intermediate (See Figure 17-5) Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003 Dehydration with POCl3 Phosphorus oxychloride in the amine solvent pyridine can lead to dehydration of secondary and tertiary alcohols at low temperatures An E2 via an intermediate ester of POCl2 (see Figure 17.6) Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Conversion of Alcohols into Alkyl Halides 3° alcohols are converted by HCl or HBr at low temperature (Figure 17.7) 1° and alcohols are resistant to acid – use SOCl2 or PBr3 by an SN2 mechanism Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Conversion of Alcohols into Tosylates Reaction with p-toluenesulfonyl chloride (tosyl chloride, p-TosCl) in pyridine yields alkyl tosylates, ROTos Formation of the tosylate does not involve the C–O bond so configuration at a chirality center is maintained Alkyl tosylates react like alkyl halides Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Stereochemical Uses of Tosylates The SN2 reaction of an alcohol via a tosylate, produces inversion at the chirality center The SN2 reaction of an alcohol via an alkyl halide proceeds with two inversions, giving product with same arrangement as starting alcohol Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003 17.8 Oxidation of Alcohols Can be accomplished by inorganic reagents, such as KMnO4, CrO3, and Na2Cr2O7 or by more selective, expensive reagents Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Oxidation of Primary Alcohols To aldehyde: pyridinium chlorochromate (PCC, C5H6NCrO3Cl) in dichloromethane Other reagents produce carboxylic acids Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Oxidation of Secondary Alcohols Effective with inexpensive reagents such as Na2Cr2O7 in acetic acid PCC is used for sensitive alcohols at lower temperatures Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Mechanism of Chromic Acid Oxidation Alcohol forms a chromate ester followed by elimination with electron transfer to give ketone The mechanism was determined by observing the effects of isotopes on rates Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

17.9 Protection of Alcohols Hydroxyl groups can easily transfer their proton to a basic reagent This can prevent desired reactions Converting the hydroxyl to a (removable) functional group without an acidic proton protects the alcohol Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Methods to Protect Alcohols Reaction with chlorotrimethylsilane in the presence of base yields an unreactive trimethylsilyl (TMS) ether The ether can be cleaved with acid or with fluoride ion to regenerate the alcohol Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Protection-Deprotection An example of TMS-alcohol protection in a synthesis Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

17.10 Preparation and Uses of Phenols Industrial process from readily available cumene Forms cumene hydroperoxide with oxygen at high temperature Converted into phenol and acetone by acid (See Figure 17.10) Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Laboratory Preparation of Phenols From aromatic sulfonic acids by melting with NaOH at high temperature Limited to the preparation of alkyl-substituted phenols Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003 17.11 Reactions of Phenols The hydroxyl group is a strongly activating, making phenols substrates for electrophilic halogenation, nitration, sulfonation, and Friedel–Crafts reactions Reaction of a phenol with strong oxidizing agents yields a quinone Fremy's salt [(KSO3)2NO] works under mild conditions through a radical mechanism Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003 Quinones in Nature Ubiquinones mediate electron-transfer processes involved in energy production through their redox reactions Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

17.12 Spectroscopy of Alcohols and Phenols Characteristic O–H stretching absorption at 3300 to 3600 cm1 in the infrared Sharp absorption near 3600 cm-1 except if H-bonded: then broad absorption 3300 to 3400 cm1 range Strong C–O stretching absorption near 1050 cm1 (See Figure 17.11) Phenol OH absorbs near 3500 cm-1 Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Nuclear Magnetic Resonance Spectroscopy 13C NMR: C bonded to OH absorbs at a lower field,  50 to 80 1H NMR: electron-withdrawing effect of the nearby oxygen, absorbs at  3.5 to 4 (See Figure 17-13) Usually no spin-spin coupling between O–H proton and neighboring protons on C due to exchange reactions with moisture or acids Spin–spin splitting is observed between protons on the oxygen-bearing carbon and other neighbors Phenol O–H protons absorb at  3 to 8 Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003 Mass Spectrometry Alcohols undergo alpha cleavage, a C–C bond nearest the hydroxyl group is broken, yielding a neutral radical plus a charged oxygen-containing fragment Alcohols undergo dehydration to yield an alkene radical anion Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003 Summary -Alcohols Synthesis Reduction of aldehydes and ketones Addition of Grignard reagents to aldehydes and ketones Protection of OH as TMS) ether Reactions Conversion to alkyl halides Dehydration Oxidation Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003

Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003 Summary - Phenols Much more acidic (pKa  10) than alcohols Substitution of the aromatic ring by an electron-withdrawing group increases phenol acidity Substitution by an electron-donating group decreases acidity Oxidized to quinones Quinones are reduced to hydroquinones Based on McMurry, Organic Chemistry, Chapter 17, 6th edition, (c) 2003